1. Field of Invention
The present invention relates to data storage generally and more particularly to data storage in a holographic medium.
2. Description of Related Art
Push-pull tracking (or sometimes “differential push-pull”) is long established in the art of optical recording. For example, this approach has been used as a tracking-error sensing method for CDs (compact disks) and other optical disks. In this context, a coherent focused beam is reflected from the surface of the disk and detected in the far field. The interference pattern between the directly reflected beam (0th-order) and the components diffracted by the optical data tracks (±1 orders) is detected by a split (two-element) photodetector to yield a highly sensitive tracking signal. ([1] Marchant, A. B., “Optical Recording: a technical overview,” Addison-Wesley, 1990, pp. 172–178.)
Typically this approach has been applied to conventional optical disks, which are reflective. In this context, the push-pull signal results from the differing optical path length traveled by the light reflecting off the grooves and ‘land’-relief pattern. These disks typically rotate while reading and writing, and the push-pull method is used to derive only a transverse (“off-track”) one-dimensional position signal. Furthermore, these applications are generally restricted to beams with relatively large numerical apertures (NA) and relatively small spot sizes (SS), thereby limiting the range of focus and the field of view. For example, in a conventional application to a CD, NA>˜0.4 and SS˜1.7 μm.
Thus, there is a need for optical tracking methods that are applicable beyond conventional applications involving one-dimensional tracking in reflective media.